Voltage Controllers

If your facility employs alternating-current (AC) induction motors that require constant speed but operate for considerable periods at very low load—meaning at less than peak efficiency—you may be able to achieve modest energy savings by installing a device known as a voltage controller.

Voltage controllers are electronic devices that sense the load on a motor and reduce the voltage applied to the motor’s terminals when the motor operates at low load. When motors operate at partial load, they draw excess magnetizing current, creating unnecessary losses in the motor core. Voltage controllers reduce this excess current, thereby reducing core losses. (Figure 1).

Figure 1: Effect of voltage on motor loss

At low loads, motor losses decrease as voltage decreases.

These controllers are most likely to be cost-effective in situations where motors operate at constant speed but spend a lot of time at very low loads. Some vendors make excessive claims about voltage controllers’ ability to save energy, but in reality, there are relatively few motor applications in which a voltage controller can save enough energy to pay for itself. Good applications for voltage controllers do exist, but it’s important to understand the conditions where they can be a viable solution. In a nutshell, a cost-effective application requires a motor that operates for very long hours at very light load. In addition, the motor must not be a candidate for alternative approaches that could save more energy, cost less, or both (such as downsizing the motor to better match the load, turning the motor off when not in use, or using a variable-speed drive). Applications meeting all of these conditions can be hard to find, but where they exist, voltage controllers can save significant amounts of energy. Cost-effective applications often include escalators and elevators, conveyors, crushers, injection molding and vacuum forming machines, lumber saws, sewing and weaving machines, machine tool spindle drives, and washing machines.

What are the options?

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Soft-start. Most voltage controllers come with a feature known as a soft-start capability. By gradually ramping up the voltage to a motor, soft-starting cuts the abrupt inrush current to less than half of the current that accompanies full-voltage starting. Benefits include reduced heat in the motor windings and lower mechanical stresses on belts, couplings, and transmissions. In addition, soft-starting can limit the interference with other voltage-sensitive equipment in a plant. Some products offer an adjustable soft-start capability that can adapt to the operator’s needs—from an instantaneous start to a ramp-up of 20 seconds or more.

Contrary to some claims, soft-starting does not reduce peak demand charge. Soft-starters don’t reduce the amount of energy necessary to start a motor; rather, they spread it over a period of seconds rather than fractions of a second. Because utility demand charges are typically calculated based on peak demand within a window of at least 15 minutes, it isn’t likely that soft-starting will reduce demand charges

Motor protection. Some models include circuits that protect motors from overcurrent and phase imbalance.

Motor monitoring. Some voltage controllers provide a digital readout of current, voltage, power factor, and power and energy consumption. You can even download the data for energy-use analysis and to assist with predictive maintenance.

Learning capability. A learning capability in some voltage controllers enables them to adapt to the characteristics of the particular motor they’re connected to.

Anticipator circuits. Some controllers have circuits that react quickly enough to load changes to prevent stalling, even with sharp increases in required torque.

How to make the best choice

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To be cost-effective, a motor must operate lightly loaded for long hours with no options for simpler or cheaper methods of energy savings. Watch out for the misleading marketing claims made by some manufacturers of voltage controllers, and pay attention to power and kilowatt-hour (kWh) savings rather than the percentage by which losses are reduced. If you happen to know the load profile of your motor’s application, you can estimate how much power you can save (Figure 2).

Single-phase motors (A) are inherently less efficient than three-phase motors (B), creating a proportionately greater opportunity for energy savings through voltage control. To evaluate potential savings, estimate the percentage of time that the motor operates at full load, follow that line up to the load level at which it runs the rest of the time, and then read across to the power savings, expressed here as a percentage of motor rating. For example, if you have a single-phase motor spending 20% of its operating time at full load and operating at one-quarter load the rest of the time, a single-phase motor controller might reduce energy use by about 25%, as illustrated on chart A.

You can use that information to get a rough estimate of a simple payback period for the device (Figure 3). The table shows a calculation based on energy savings alone.

Figure 3: Sample voltage controller payback calculation

This table illustrates a payback calculation for a voltage controller installed on an escalator at Caesar’s Palace in Las Vegas, Nevada. In this example, the voltage controller will pay for itself in about two years.

Some things to keep in mind when choosing a voltage controller:

Pick a controller that matches the motor. Voltage controllers are available for single-phase and three-phase motors in the full range of National Electrical Manufacturers Association enclosures (the gray boxes that protect them from weather), with voltage ratings up to 600 volts and in sizes up to 1,000 hp.

Look for devices that produce low total harmonic distortion (THD). The generation of harmonics within an AC induction motor circuit can cause both an increase in running temperature and interference with other electrical equipment.

Pay attention to your average load. Voltage controllers can only save appreciable energy in motors that run at low load or completely unloaded most of the time they’re operating.

Consider your hours of operation. A motor must run at low load for extended periods to create a viable energy-saving opportunity from a motor voltage controller; if your application only runs a few hours per day, the savings may not be high enough for it to pay for itself in a reasonable time frame.

There are some applications in which voltage controllers just aren’t appropriate or practical:

Downsizing. If a motor has a low average load factor and never approaches its full rated power, it may simply be oversized for the function it’s performing. In such cases, replacing the motor with a smaller, more-efficient model that will operate at an appropriate load point could easily yield greater life-cycle savings at lower cost than a voltage controller—particularly if the existing motor is nearing the end of its useful life. However, if the motor is occasionally required to supply 75% or more of its rated power, downsizing is not an option, and the voltage controller may be cost-effective.

Load cycling. Another alternative to a voltage controller is to switch the motor off when it’s not in use. Many motors that serve low average loads operate completely unloaded most of the time but are needed intermittently to provide full rated power, or something close to it. If the load for such applications is predictable, or if it can be anticipated—either automatically with sensors or by human intervention—it may be possible to shut the motor down entirely between loads.

Variable speed. For loads that can accommodate variable-speed operation, variable-frequency drives (VFDs) can reduce energy consumption at low load much better than voltage controllers can. For pump and fan applications, reducing motor speed can dramatically cut energy consumption. For other types of loads, energy savings are not nearly as dramatic, and VFDs can actually use more power in traction-type loads. If a VFD can be used, a voltage controller is almost certainly shouldn’t.

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.

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